Laser Combs for Astrophysics ("Astro-comb")
We have developed a new technique using laser frequency combs to improve the accuracy and stability of wavelength calibration of astrophysical spectrographs by up to two orders of magnitude. This "astro-comb" will provide a key advance in the resolution of changes in astrophysical Doppler shifts and redshifts, and thus may allow the discovery of Earth-like planets and new measurements of astrophysical dynamics relevant to cosmology.
Both of the leading techniques to find exoplanets rely on the planet's very small effect on its star. The radial velocity technique measures the star's "wobble" (and hence Doppler shift) due to the planet's gravitational pull as it circles; while the transit technique measures the dimming of a star's light as a planet passes in front of it. With current technology, both of these techniques can identify relatively large planets that have a noticeable effect on their star. Detection of the very small Doppler shifts (<10 cm/s) induced by smaller, rocky planets, like Earth and Mars, will require improved wavelength calibration that can only be provided by astro-combs.
The astro-comb uses a mode-locked laser frequency comb and a Fabry-Perot filtering cavity, linked to an atomic clock, to provide an accurate and stable wavelength standard well-suited to astrophysical spectrographs and against which light from a star can be measured. We have developed prototype astro-combs and tested them successfully at astronomical observatories. We are currently developing an astro-comb to search for Earth-like planets as part of the HARPS-North project.
- Optimization of filtering schemes for broadband astro-combs. G. Chang, C. H. Li, D. F. Phillips, A. Szentgyorgyi, R. L. Walsworth and F. X. Kärtner. Optics Express, 20, 22, 24998-25013 (2012).
- Conjugate Fabry–Perot cavity pair for improved astro-comb accuracy. C.H. Li, G. Chang, A. G. Glenday, N. Langellier, A. Zibrov, D. F. Phillips, F. X. Kärtner, A. Szentgyorgyi, and R. L. Walsworth. Optics Letters, 37, 15, 3090-3092 (2012).
- Calibration of an astrophysical spectrograph below 1 m/s using a laser frequency comb. D.F. Phillips, A.G. Glenday, C.H. Li, C. Cramer, G. Furesz, G. Chang, A.J. Benedick, L.J. Chen, F.X. Kärtner, S. Korzennik, D. Sasselov, A. Szentgyorgyi, and R.L. Walsworth. Optics Express, 20, 13, 13711-13726 (2012).
- Broadband dispersion-free optical cavities based on zero group delay dispersion mirror sets. L.J. Chen, G. Chang, C.H Li, A.J. Benedick, D.F. Philips, R.L. Walsworth and F.X. Kärtner, Optics Express 18, 23294 (2010).
- In-situ determination of astro-comb calibrator lines to better than 10 cm/s. C.-H. Li, A. Glenday, A. J. Benedick, G. Chang, L.-J. Chen, C. Cramer, P. Fendel, G. Furesz, F. Kärtner, S. Korzennik, D. Phillips, D. Sasselov, A. Szentgyorgyi, and R. Walsworth. Optics Express 18, 13239-13249 (2010). arXiv:1006.0492.
- Visible Wavelength Astro-comb. A.J. Benedick, G. Chang, J.R. Birge, L-J Chen, A.G. Glenday, C-H Li, D.F. Phillips, A. Szentgyorgyi, S. Korzennik, G. Furesz, R.L. Walsworth and F.X. Kärtner. Optics Express 18, 19175 (2010).
- Toward a Broadband Astro-comb: Effects of Nonlinear Spectral Broadening in Optical Fibers. G. Chang, C-H Li, D.F. Phillips, R.L. Walsworth, and F.X. Kärtner. Optics Express 18, 12736-12747 (2010).
- A laser frequency comb that enables radial velocity measurements with a precision of 1 cm s-1. C.-H. Li, A.J. Benedick, P. Fendel, A.G. Glenday, F.X. Kartner, D.F. Phillips, D. Sasselov, A. Szentgyorgyi and R.L. Walsworth, Nature 452, 610-612 (2008) 0804.0955.
- With a coarse-tooth comb. G. Walker, Nature 452, 538 (April 3, 2008).
- Easy ways to other Earths. E. Hand, Nature 452, 514 (April 3, 2008).
- Laser precision added to search for new Earths. A. Powell, Harvard website (April 2, 2008).